Alkaloids

Alkaloids were formerly defined as basic, nitrogen-containing plant substances, but examples have also been found in fungi, marine organisms, amphibians, insects and even mammals. There are over 10,000 plant alkaloids known, very varied in structure, from simple ones like coniine to complex structures like strychnine and reserpine (Figure 9.1). The biosynthesis of alkaloids in plants has received a lot of study. Less is known about the biosynthesis of the much smaller number of insect alkaloids, except for a few examples, like the coccinellines, adaline and epilachnine (Chapter 3).

All the available evidence suggests that plants make alkaloids to deter predators. Some, like the tobacco alkaloids are strongly toxic to insects. Nicotine, anabasine and other related alkaloids are produced in the roots of the tobacco plant and translocated to the leaves. Nicotine is certainly toxic to most insects. Formerly, a crude preparation of nicotine was used commercially as an insecticide, but the tobacco hornworm {Manduca sexta) (Plate 15) has adapted itself so that its larvae feed only on tobacco strychnine h3co'

h3co'

o och3

och, strychnine

Figure 9.1 Some examples of plant alkaloids. Coniine is from the leaves and seeds of hemlock and very toxic to motor nerves. Strychnine, from Strychnos nux-vomica, is a central nervous system and respiratory stimulant, also very toxic. Reserpine, from Rauwolfia serpentina roots, is used clinically against hypertension and formerly as a tranquillizer leaves, and others, like the cigarette beetle Lasioderma serricorne live only on dried tobacco leaves.

Nearly half of all plant families have at least one species that contains alkaloids, but of about 10,000 plant genera, less than 10% are known to make alkaloids, so the distribution is very patchy. Alkaloids are particularly common in the Leguminosae (peas and beans) and the Solanaceae (tomato, potato and tobacco).